This invention relates to the field of integrated circuit processing. More particularly, this invention relates to removing a photoresist mask following a metal etching process.
Integrated circuits are typically electrically interconnected via electrically conductive traces, such as metal lines. Aluminum has been an often used component in such conductive traces because of its low cost and good conductivity. Typically, other metals and metallic compounds are deposited in conjunction with an aluminum layer to form a conductive stack of layers that are patterned to form the conductive traces. For example, a typical conductive stack may include layers of materials such as titanium, titanium nitride, aluminum, and copper. Because the conductive stack is often primarily formed of aluminum, it is sometimes referred to hereinafter as the aluminum layer. However, it is appreciated that there are typically other metal and metal compound layers formed with the aluminum layer.
In a typical process for forming the aluminum traces, a photoresist mask is patterned on the aluminum layer, with photoresist remaining on those portions of the aluminum layer that are to form the conductive traces, and the photoresist removed from those portions of the aluminum layer that are to be removed. Once patterned, the aluminum layer is etched, such as with chlorine (Cl2) and boron trichloride (BCl3) gasses in a reactive ion etch. Once the aluminum layer is etched, it is desirable to remove the photoresist mask so that subsequent processing can be accomplished. The photoresist mask is typically removed in an oxygen gas plasma, in which the photoresist mask is oxidized. This process is commonly referred to as ashing.
However, during the etch process, a polymer layer is typically formed around the exposed surfaces of the photoresist mask and the sidewalls of the aluminum traces. The polymer layer includes reaction products from the etching process, such as carbon, aluminum, oxygen, chlorine, nitrogen, silicon, titanium, and some amount of entrained chlorine gas. Unfortunately, the polymer layer tends to reduce the effectiveness of the ashing process, because it tends to shield the photoresist layer from the oxygen plasma.
Therefore, an intermediate process is typically performed between the etching process and the ashing processing. The goals of the intermediate process are to modify the polymer layer so that the photoresist layer is more easily removed during the subsequent ashing process, and to ensure that the sidewalls of the aluminum layer are sufficiently passivated. This intermediate process is often referred to as a passivation process.
During the passivation process, a microwave water vapor plasma is formed and introduced into the reaction chamber, wherein resides the substrate on which the aluminum traces have been formed. The water vapor plasma softens the polymer layer that coats the photoresist mask and the sidewalls of the aluminum traces, and tends to remove most of the entrained chlorine gas from the polymer layer. The substrate is typically heated on a chuck during the passivation process, to aid in the removal of the entrained chlorine gas. Some amount of the photoresist layer may also be removed in the passivation process.
However, this passivation process only softens and does not completely remove the polymer layer, and thus the subsequent ashing process tends to not be completely effective in removing the photoresist layer, unless an excessively long ashing process is used. Such an excessively lengthy ashing process tends to reduce the throughput of the ashing process, and thus tends to increase costs. Furthermore, the passivation process also tends to introduce other problems.
For example, sometimes the water vapor condenses in the vapor delivery line and is not completely dissociated in the upstream microwave plasma, and water reaches the polymer layer. The water then tends to react with the entrained chlorine gas in the polymer layer to form a highly corrosive solution that attacks and degrades the aluminum traces. Further, the thermal energy from the block heater on which the substrate is disposed tends to cause the polymer layer to form a crust on its outside layer. The crust tends to be more resistant to the ashing process, and thereby further reduces the effectiveness of the ashing process.
What is needed therefore, is a passivation and ashing process that more effectively removes the photoresist layer while adequately protecting the aluminum traces from corrosion and damage.
The above and other needs are met by a method for forming a conductive trace on a substrate. The conductive trace is patterned with a photoresist mask and etched, thereby forming a polymer layer on a top surface and sidewalls of the photoresist mask and on sidewalls of the conductive trace. The polymer layer contains entrained chlorine gas. The substrate is heated on a chuck in a reaction chamber. A remote plasma is generated from ammonia gas and oxygen gas. The substrate is contacted with the ammonia and oxygen plasma, thereby withdrawing a substantial portion of the entrained chlorine gas from the polymer layer. A radio frequency potential is applied to the chuck on which the substrate resides, thereby creating a reactive ion etchant from the ammonia and oxygen plasma in the reaction chamber and removing the polymer layer from the top surface of the photoresist mask. The photoresist mask is thus exposed, and then removed in an ashing process.
By having no water vapor in the remote plasma used to withdraw the entrained chlorine gas, no corrosive chlorine solution is produced and thus no damage to the conductive trace occurs. Further, by applying the radio frequency potential to the chuck during the process, the polymer layer on at least the top surface of the photoresist mask is removed, thereby allowing for a faster and more complete removal of the photoresist mask during the ashing process. In this manner, a higher quality conductive trace is formed in a shorter period of time, thus improving the quality and reducing the cost of the integrated circuits that include such conductive traces.
In various preferred embodiments of the invention, the conductive trace comprises aluminum, which is etched with a reactive ion etch of chlorine and boron trichloride. The remote plasma is preferably generated using a microwave generator at a power of between about 500 watts and about 3000 watts, but may alternately be generated by an inductive coupling power source at a power of between about 200 watts and about 1500 watts.
The remote plasma may additional include nitrogen gas, hydrogen gas, argon gas, and a fluorinated hydrocarbon gas, where the fluorinated hydrocarbon gas is at least one of CxFy, CxHyFz, and NF3. Most preferably, the ratio of the ammonia gas to the oxygen gas in the remote plasma is adjusted to be between about 1:100 and about 10:1 to reduce and preferably minimize oxidation of the sidewalls of the conductive trace.
The exposed photoresist mask is preferably removed by ashing the photoresist mask with either a remotely generated microwave oxygen plasma or the remotely generated ammonia and oxygen plasma.